Semiconductor Dies; In General
Improved methods for miniaturization of semiconductor dies have permitted the integration of millions of transistor circuit elements into a single silicone embodied circuit. Such a circuit is typically referred to as an integrated circuit chip or a semiconductor die.
Semiconductor dies are created from a silicon wafer through the employment of various etching, doping and depositing steps that are well known in the art. Ultimately, the semiconductor die may be packaged by encapsulating the semiconductor die to form an "integrated circuit package" having a variety of pin-out or mounting and interconnection schemes. An integrated circuit package is hereinafter referred to as an "IC package." More sophisticated IC packages have been developed for very large scale integration ("VLSI") semiconductor dies that can accommodate the increased number of external connections required with an electronic system.
PGA and BGA Packaging
VLSI IC packages having high connection capacity are, for example, pin grid array ("PGA") and ball grid array ("BGA") type packages. Both PGA and BGA type packages, including adaptations thereof for surface mount and hybrid applications, employ one or more printed wiring boards (hereinafter referred to as "PWBs"). Such PWBs consist of, for example, polyimide, glass reinforced epoxy, ceramics, or other materials known to those skilled in the art of fabricating very large scale IC packages. Some of the PWBs have material cut out from the middle, which when laminated together, form a cavity in which a semiconductor die may be placed.
The PGA and BGA packages differ mainly in that a PGA package utilizes conductive metal pins that may be either soldered to a system printed circuit board or inserted into a matching socket which is already soldered to the system printed circuit board. In contrast, BGA packages utilize "solder balls" instead of metal pins. The solder balls of a BGA package reflow to connection points on a system printed circuit board when heated to a certain temperature, thus, electrically connecting the circuitry within the BGA IC package to an external electronic system.
Connections are made from bond pads of a semiconductor die to contact pads of PWBs, and then to conductive traces of the PWBs. The conductive traces further connect to either the connection pins or connection solder balls of a PGA or BGA package, respectively. Thus, the PGA or BGA package is a miniature multi-layer printed circuit board system containing the semiconductor die and forming a housing for protection of the die. Examples of VLSI IC packages are more fully illustrated and described in co-pending U.S. patent application Ser. No. 07/917,894 entitled "Ball Bump Grid Array Semiconductor Packages" by Michael Rostoker, Chok J. Chia, Mark Schneider, Michael Steidl, Edwin Fulcher and Keith Newman, filed on Jul. 21, 1992, and assigned to LSI Logic Corporation, the disclosure of which is incorporated by reference herein for all purposes.
IC Package Lids
A semiconductor die is ordinarily encapsulated so as to insulate the semiconductor die from adverse mechanical, chemical, electrical, and thermal environments. An encapsulant can be employed to completely fill the cavity wherein the semiconductor die is disposed. Such an encapsulant, often referred to as a "glob-top" encapsulant, is usually a highly viscous liquid or paste. Glob-top encapsulants are typically filled thermoset plastics that are based mostly on epoxy or silicone technology.
Glob-top encapsulants, such as epoxy, have relatively high viscosities. Furthermore, such encapsulants can be unevenly dispensed and are susceptible to shrinkage when cured. As a result, a glob-top encapsulant does not provide an even surface. A planar lid is employed to cover the unevenly distributed encapsulant and thus ensure that the top and bottom surfaces of an IC package are parallel with one another. Accurate testing and handling of an IC package is typically dependent on the existence of parallel top and bottom surfaces of the IC package.
Currently known technology utilizes a full body aluminum lid to seal a glob-top encapsulant within a cavity of an IC package. A full body aluminum lid covers the top surface of the IC package wherein the encapsulant filled cavity is located, as well as wraps over the sides of the top surface of the IC package in order to secure the lid to the IC package. A full body aluminum lid thus precludes decoupling chip capacitors from being advantageously disposed around the periphery of an IC package.
Full body metal lids, which are currently employed to seal a glob-top encapsulant in a cavity of an IC package, possess several attributes that are desirable of improvement. First, the coplanarity and parallelism of an IC package that employs a full body metal lid is desired to be improved. This can be accomplished by providing a lid that reduces the volume of glob-top encapsulant in the cavity of an IC package. By reducing the volume of glob-top encapsulant, the IC package substrate structure (such as PWBs) is allowed to dominate and thus reduce the level of distortion and warpage of an IC package. This, in turn, leads to improved coplanarity and parallelism of the IC package. Second, the reliability of an IC package that employs a full body metal lid is desired to be improved. Placement of a full body metal lid can trap air voids within a glob-top encapsulant during fabrication. Moisture may collect in the air voids and result in mechanical failures such as cracking and catastrophic fatigue, as well as other failures such as corrosion. Thermal performance of an IC package may also be diminished when air voids exist. A lid that reduces the likelihood of air voids within a glob-top encapsulant is thus desirable. Third, a full body metal lid fails to deter the undesirable lateral flow of encapsulant from a cavity. A lid that adequately deters such lateral flow is desired.
What is needed is a lid, which can be easily manufactured in a cost effective manner, that overcomes the aforementioned deficiencies of currently utilized full body metal lids.